1. Field of the Invention
This invention relates to radio navigation systems generally, and more particularly to radio navigation systems which radiate simulated Doppler signals which effectively angle-code the transmissions for the benefit of a remote station deriving angle information therefrom.
2. Description of the Prior Art
The derivation of navigational information using the (simulated) Doppler technique as known in the prior art and as applied to a transmitting radio beacon basically involves movement of a source of radio frequency (typically of microwave frequency of 1 or more GHz) along an array of antennas. This movement or scan is simulated in practical systems by the sequential switching of the radio frequency source to each element in such a way as to represent constant velocity movement of the source along the axis of the array. In this way a simulated Doppler effect is produced which is unrelated to Doppler effect due to relative beacon/target actual motion. Two such arrangements are described in U.S. Pat. Nos. 3,626,419 and 3,670,337.
Depending on the cosine of the angle subtended by a remote station (airborne) receiver with respect to the array axis, a phase shift is imparted to the remotely received signal. The change of phase, as the contribution of each element follows the last, is observed by the remote receiver (and all other such receivers in the service sector of the beacon at appropriately different values according to their respective angles). In a given receiver, the total of these contributions constituting a scan entity, is analyzed by suitable processing means to obtain the angle (bearing or elevation from a horizontal axis or a vertical axis array respectively).
In practice two sources are used for the transmission of information in this Doppler navigation beacon format, the beat between them being extracted for derivation of Doppler coding at the remote location. As is well known in the germane prior art, the rationale for the use of two sources is the need for elimination of the effects of instabilities due to transmitter, receiver and aircraft motion. The two sources have been referred to as the reference and commutated (main array or information) components, but it is clear that the phase of the beat component at a remote receiving location is equally influenced by the phase of either signal, so that movement of either of these sources is equally effective in constituting a scan sequence.
It is also possible, therefore, to generate such a scan by commutation of both sources according to a predetermined program. Two such proposals are contained in U.S. Pat. No. 3,953,854 and U.S. Pat. application Ser. No. 574,853 filed May 6, 1975, now abandoned.
In addition to the Doppler Beacon application, the Doppler technique may be applied to a receiving array of antennas of n radiating elements, as described in U.S. Pat. No. 3,924,236, in what may be termed an "inverse" Doppler system. In that type of system successive r.f. pulse signals are each received simultaneously by all the elements of the array, which for each received pulse, induce a series of n simultaneous signals; the phase shift between successive signals in each series being determined by the angle of arrival of the corresponding signal with respect to the array axis.
The received signals may have been transmitted by an aircraft in the service sector of the array, or the aircraft may reflect pulse signals from separate illumination as in some radar systems.
Subsequent processing of each of the series of n signals involves the use of a second frequency, offset from the information (Doppler frequency coded) signal series, generated at the receiving station for mixing with the information signal series to extract the Doppler beat freqency which is subsequently processed to determine the phase shift and hence the angle of arrival of the received pulses. Such devices, as a class, are sometimes referred to as synthetic aperture devices.
In both aforementioned types of Doppler systems, multipath signals may occur, by reflection, and where such multipath signals are not sufficiently spectrally removed in angular coding to be rejected by the tracking processor, errors may be introduced into normal angle indication. In both systems, the amplitude and sign of the resultant error is influenced by the relative phase (at information frequency) of desired (direct path) and multipath signals.
It is the aim of processes designed to reduce the effect of multipath to vary the relative phase of desired and undesired components by an amount which, over a scan or a time slot (predetermined number of scans), is able to produce some useful averaging of the basically phase-conscious error. Relative phase difference may in general be introduced between direct and multipath signals by a change in relative path length. One way of achieving this is by diversity, i.e. movement at the antenna array.
In co-pending U.S. Pat. application Ser. No. 597,369, filed July 21, 1975, there is described a co-ordinate array diversity scheme, applicable both to a Doppler transmitting beacon and to an inverse Doppler direction finding receiver. In that prior art device there is utilized an antenna scanning sequence which introduces a first order of diversity, which for example, in a vertical scanning transmitting Doppler beacon for elevation guidance, basically involves the simultaneous realization of the necessary constant velocity vertical separation of two radio frequency sources during each scan to generate the necessary information signal by direct path propagation to a receiver, with pseudo-random horizontal movement of the two sources at successive steps during each scan, the two sources moving sideways together to remain vertically aligned but along successively different vertical axes.
The results of prior art diversity and other multipath rejection system still leave something to be desired in respect to their ability to reject undesired signals. The manner in which the present invention significantly improves upon this situation will be evident as this description proceeds.